Embodiments of the present invention relate to a blade, and in particular to a fan blade for a gas turbine engine.
A fan of a gas turbine engine comprises a fan rotor and a number of circumferentially spaced radially outwardly extending fan blades secured to the fan rotor. The fan is surrounded by a fan casing, which defines a fan duct, and the fan casing is arranged to contain one or more of the fan blades in the unlikely event that a fan blade becomes detached from the fan rotor.
If a fan blade becomes detached from the fan rotor, for example due to impact with a large foreign body such as a bird, the detached fan blade strikes a main fan casing containment region and generally progressively breaks up under a buckling action. Fan blades conventionally increase in strength from the tip to the root and at some position between the tip and the root the remaining portion of the fan blade, including the root, no longer buckles. The remaining portion of the fan blade has substantial mass and is accelerated by the trailing blade until it impacts a rear fan containment region of the fan casing.
It is necessary to provide additional material to the rear fan containment region of the fan casing to contain the remaining portion of a detached fan blade. The additional material may be in the form of an increase in thickness, the provision of ribs, honeycomb liners etc, the impact energy being dissipated by plastic deformation of the additional material. However, these methods of protecting the rear fan containment region are disadvantageous as they add weight to the gas turbine engine.
One approach taken to the above problem is defined in U.K. patent publication no. 2399866 (Rolls-Royce). Apertures are provided in the root which extend through that root. The apertures create beneficial deflection upon impact such that there is a reduced load placed upon the rearward portions of the fan casing. It is therefore less necessary to provide additional reinforcement in the casing to resist remaining portions of the fan blade as described above. Essentially, by allowing deformation there is a reduction in the energy transferred to the casing by encouraging break up of the root fragment about the apertures, or at least flexing as described. Unfortunately this approach requires use of intrusive machinery within the blade in order to form apertures which extend through the root generally to the blade cavity between the surfaces of that blade.
The problem with respect to machining processes such as drilling or otherwise to form the apertures is the associated risk of tool breakage in, by this stage, a relatively high value component. Furthermore, it will also be understood that break out of the aperture into the cavity formed in the blade is hard to design and control. Additionally, generally the cavity is no longer sealed by the aperture passing through the root to it and therefore generally a further operation is required in order to prevent fluid ingress to the cavity in use. Finally, it will be understood that if the cavity between the surfaces of the blade extends to a relatively low position in the root, that is to say the root is relatively thin, the introduction of apertures may create particular problems in this highly stressed region of the blade.